For example, take an urban block that is, for the sake of argument, 90% Roman Catholic. In the east, there would be a heavy ethnic predominance toward one type, in the midwest, somewhat mixed. Nonetheless, as the calendar ticked round, nearly everybody on the block was observing particular cultural traditions on the same day. The adults and kids talked about it, prepared for it, and would even mix households and celebrate some things together.
Move 40% of that block out to the new suburbs surrounding Lake McMansion. All of a sudden, you don't have that common purpose and common event out at the lake - some years, they even start to forego it for other activities. And in the old neighborhood, that old ethnic and religious cohesiveness is irreparably broken, even for those who are left.
Actually, your arguments are well taken, and they are virtually identical to what E. Michael Jones has been saying in several articles and books. He believes that in many cases the highway projects were deliberately steered to break up ethnic voting blocs in major cities -- usually Catholic voting blocs, but sometimes other groups as well.